Effect Of Different Heat Treatment On The Physicochemical Properties And In Vitro Digestibility Of Starches From Normal Rice And Waxy Rice

Rice is one of the most important crops in China,and starch is its main nutrient.At present,there are few comparative studies on the effects of heat treatment with different moisture content on the physicochemical and digestive properties of rice starch.In the present paper,normal rice with higher amylose content and waxy rice with lower amylose content were selected,and their physicochemical properties and in vitro digestibility after dry heat treatment(with 8%moisture content),heat-moisture treatment(with 20%and 30%moisture content)and gelatinization treatment(with 70%moisture content)were compared.The results were as follows:(1)Effects of different heat treatment methods on the molecular structure and crystallinity of starch.The shape of the starch particles of both normal rice and waxy rice was polygonal and the surface was smooth.After heat treatment,pores and depressions appeared on the surface of the particles.The gelatinization treatment showed significant damage with the damage degree increased to 40.77-42.73%and the starches were bonded to large particles.The amylose content of the modified samples increased except for the gelatinization treatment of waxy rice starch(WR-70%).The highest content of amylose(26.03%)occured in the heat-moisture treatment in 30%moisture content of rice(NR-30%).The molecular structure and crystallinity of the starch changed after the heat treatment,and the short-range molecular order and crystallinity increased after dry heat treatment,while decreased after heat-moisture treatment.After gelatinization treatment,the Fourier transform infrared spectroscopy(FTIR)absorption peak of starch changed significantly with the short-range molecular order and crystallinity also decreased significantly.The gelatinization of normal rice and waxy rice starch was endothermic.The thermodynamic curve parameters(To,Tp,Tc)of dry heat-treated rice starch reduced to different extents,and gelatinization enthalpy increased while the results of heat-moisture treatment were completely opposite.The gelatinized waxy rice starch did not show a peak of absorption,indicating that the starch was completely gelatinized.The moisture content of starch was positively correlated with starch damage degree and gelation temperature parameter,but negatively correlated with short-range molecular order,relative crystallinity and gelatinization enthalpy after heat treatment.(2)The effect of different heat treatment methods on functional properties of starch.The native waxy rice showed better freeze-thaw stability and was suitable for making frozen or canned food.The starches showed high water loss rate after different heat treatments,indicating that different heat treatment resulted in a decrease in starch freeze-thaw stability.After the heat-moisture treatment and the gelatinization treatment,the transparency of the normal rice and the waxy rice starch paste increased to 62.59-62.77%,and they can be applied to products with higher transparency requirements.Different heat treatment methods Ied to an increase in the solubility of normal rice and waxy rice starch,and the swelling power reduced,so that it can be used as a pre-gelatinized starch,and products that needed to limit a starch expansion effect such as noodles,rice noodles,sweet dumplings,etc.The heat treatment also increased the water absorption capacity(WAC)and oil absorption capacity(OAC)of the starch,but a lower oil absorption capacity was observed after the heat-moisture treatment(30%moisture content)and the gelatinization treatment.The moisture content in the system during heat treatment was significantly positively correlated with the solubility and transparency of the treated starch,but negatively correlated with the swelling power and WAC.(3)Effects of different heat treatment methods on in vitro digestibility.After dry heat treatment,the content of rapidly digestible starch(RDS)and slowly digestible starch(SDS)decreased,while the content of resistant starch(RS)increased.The hydrolysis index and glycemic index decreased during the heat-moisture treatment at 20%moisture content and the heat-moisture treated starch had less RDS,RS,and more SDS than the counterparts.Compared with other modification methods,the gelatinization treatment had a great influence on the in vitro digestibility of starch.The rate and the degree of starch digestibility increased and the final digestibility of the two starches was about 80%.